We put forward an approach to deriving and applying human error tolerance requirements. Such requirements are concerned with the response of a system to errors introduced by human operators. The approach provides a means by which operators' tasks can be described and analysed for likely errors and the impact of these errors on system safety can be explored. The approach, based on previous work by the same authors (P. Wright et al., 1994), uses a software engineering notation to provide the bridge between operator models and systems engineering concerns. The approach is extended to include a more refined understanding of the processes that contribute to human error. The operators' process in achieving goals is understood in terms of structured tasks. With this additional apparatus we are able to capture a more complex set of human error forms.